As a bearing steel, high carbon chromium bearing steels such as SUJ2 and the like stipulated in JIS G 4805 (1999) have been used conventionally for the material of bearings used in various fields of automobiles, various industrial machines and the like. However, bearings have a problem that, because they are used in a severe environment as the case of the inner ring, outer ring, rolling element and the like of the ball bearing, roller bearing and the like whose pressure of contacted surface is very high, fatigue fracture is liable to occur from a very fine defect (inclusions and the like), and frequent maintenance (replacement, inspection and the like) is required in order to prevent the fatigue fracture. With respect to this problem, improvement of the bearing steel has been tried in order to increase the rolling-contact fatigue life and to reduce the number of times of the maintenance described above.
Conventionally, extension of the life of the rolling-contact fatigue life has been improved by reducing non-metal inclusions (Patent Literature 1 and Patent Literature 2 for example). However, to reduce the non-metal inclusions industrially is approaching the limit.
Therefore, as a method for improving the life from other aspects, reduction of banded segregation (Patent Literature 3 for example) and suppression of formation of carbide in the center segregation section (Patent Literature 4 for example) have been proposed. Also, in Patent Literature 5, improvement of the rolling-contact fatigue life by miniaturizing the grain size has been disclosed.
In the Patent Literature 3, variation of the hardness caused by banded segregation is reduced by lowering the rolling temperature and increasing the press forging ratio (60 or more), and, in the Patent Literature 4, formation of macro carbide is suppressed by extending the soaking treatment time, and the life is improved. However, because these methods had restriction in rolling method and rolling size, they could not be regarded as methods having high degree of industrial freedom, and the life improvement effect could not necessarily be regarded to be enhanced to a desired level. Further, in the Patent Literature 5, although the grain size is miniaturized by extending the spheroidizing treatment time and applying induction hardening, extension of the spheroidizing treatment deteriorates the productivity and is limited to the induction hardening treatment, and therefore the method cannot be regarded to have high degree of industrial freedom.
Also, in Patent Literature 6, in order to make the polishability in working into a product shape excellent and to stably obtain excellent rolling-contact fatigue life, the size and density of Al-based nitrogen compound dispersed in steel and the size and area ratio of cementite are stipulated in particular. Also, in Patent Literature 7, in order to obtain a wire rod for bearing that does not cause wire breakage even when strong drawing work is effected so that the drawing reduction ratio exceeds approximately 50% (or 70%) and is suitable to strong drawing work, it is proposed to make the average equivalent circle diameter of cementite after spheroidizing annealing and the standard deviation thereof a constant or less and to suppress the variation by controlling the production condition.
However, all of them are for controlling the form of the precipitates, improvement effect of some degree can be seen, but in order to further improve the rolling-contact fatigue life, it seems necessary to study further from other aspects.